John Calvin Commentary Psalms 102:20

"To hear the sighing of the prisoner; To loose those that are appointed to death;" — Psalms 102:20 (ASV)

To hear the groaning of the prisoner. Here the prophet repeats once more what he had previously touched upon concerning prayer. He does this to stir up again the hearts of the godly to engage in that exercise, and so that after their deliverance they might know it had been granted to their faith, because, depending on the divine promises, they had sent their groanings up to heaven. He calls them prisoners; for although they were not bound in fetters, their captivity resembled a most rigorous imprisonment. Indeed, he affirms a little later that they were devoted to death, to help them understand that their life and safety would have been altogether hopeless, had they not been delivered from death by the extraordinary power of God.